Printed circuit boards manufactured for military and commercial users are frequently coated with a protective film, referred to as a "conformal coating", to avoid degradation in electrical performance due to deleterious environmental effects, e.g., water, dust, and air-borne pollutants. The most damaging of the adverse environmental effects is generally recognized to be humidity. Humidity corrodes electrical components, lowers resistance between conductors, accelerates high voltage breakdowns, and even causes destructive short circuits. In addition to protecting the printed circuit board from air-borne contaminants, conformal coatings protect against contaminants inadvertently deposited by persons handling the board, e.g., grease, hair, cosmetics, and fingerprints.
Conformal coatings are generally coatings of synthetic resins which form a tough, non-conductive, water-impervious film. Conformal coatings are applied evenly and thinly over the entirety of the printed circuit board. Generally, these coatings are from about 0.005 to about 0.0005 inch thick. For economic reasons conformal coatings are generally applied by machine using mass production techniques. The most popular of these techniques is spraying the coating onto the board. Although several conformal coating systems are available, each serves primarily the same function, and for purposes of the present invention they are indistinguishable.
Conformal coating material if misapplied will damage components attached to the printed circuit board, rendering the board useless. For example, if the conformal coating material were allowed to enter into and coat the electrical contacts of an unoccupied integrated circuit socket, that socket would become permanently inoperable since it could no longer become electrically connected to an integrated circuit. Depending on the circuit path in which the useless socket is situated, the entire printed circuit board might become useless.
Several non-electrical components on a board, such as pivoting ejector levers and toggle switches, are movable. Therefore, such components must also remain uncoated if they are to remain functional. Other components such as fuses and EPROMs cannot be coated because they must be removable for replacement purposes in the future. Accordingly, there are several electrical and non-electrical components which are used on printed circuit boards and which must remain uncoated if the board is to function properly.
Masking techniques are typically used to avoid the conformal coating of those components on a printed circuit board which are to remain uncoated. The most popular of these masking techniques uses adhesive masking tapes. According to this technique, a length of adhesive tape is tailored to conform to a surface of a selected component, and then manually applied to the surface of the component. Since a single printed circuit board often includes multiple components requiring masking, tape masking is a labor-intensive, time-consuming, and expensive technique. For example, masking a single component on one board may take an experienced technician up to 30 minutes. Multiply the period required to mask one board by the hundreds of thousands of boards coated annually in the United States alone, and it is apparent that the cost of masking printed circuit boards with tape is substantial. Furthermore, because the size of the components masked is generally small, and because printed circuit boards themselves have become so crowded, applying these tapes is a tedious task, and errors often occur, leaving critical components unmasked or only partially masked. Accordingly, numerous boards are ruined, or must be extensively reworked.
A further problem associated with the tape masking of components, separate and apart from applying the tape, is the subsequent removal of the tape. Often, because the tapes are sealed beneath the conformal coating, the use of a sharp instrument, such as a razor, is required. This can result in damage to the board. When considering that a conformal coating is only 0.0005 to 0.005 inch thick, it becomes apparent that a steady and gentle hand is required to cut through the thin coating to retrieve the pliable tape beneath without damaging the board.
The additional step of removing the tape also increases manufacturing costs by raising labor costs. Removing the coated tape can require a great deal of additional time. Again, multiplying the time required to remove the coated tape from one board by the total number of boards coated results in significant added costs.
A further problem in the conformal coating and manufacture of printed circuit boards is static electrical discharge. Electrical integrated circuit chips are extremely susceptible to the ravages of static electrical discharge. For example, when subjected to static electrical discharge, integrated circuits can be destroyed, or the parameters of the integrated circuits can be significantly altered. It has been found that static electrical discharges sufficient to destroy or alter integrated circuits can be produced through handling, transporting, masking, or coating the printed circuit board.
Damage from static electrical discharge often occurs during the conformal coating process. Electrostatic charges can be generated by the conformal coating process itself as the liquid droplets pass through a spray nozzle and then the air before being deposited on the board. The adhesive masking tapes also generate large static charges when peeled from their reels, and when removed from the surface of the masked component. These electrostatic charges can be in the tens of thousands of volts. Unfortunately, these charges are often discharged through the circuit board causing damage to the susceptible components thereon. For example, during the application or removal of masking tape, a technician or tape carrying an electrostatic charge may contact an electrical conductor on the board, causing a discharge which damages one or more integrated circuits connected to that conductor.
Manufacturers recognize the potential for damage from static electrical discharge during the application and removal steps of tape masking. To minimize such damage, tapes are often bathed in ionized air before and during their application, and also during their removal. However, air ionization is not always sufficient to reduce static charges quickly enough to prevent static discharge to the components in contact with the tape.
Another attempt to solve this problem has been the use of tape with conductive adhesive. The conductive adhesive facilitates the removal of charges generated during the application or removal process by conducting these charges safely to ground. However, problems exist with the conductive adhesive approaches attempted to date. Conductive adhesive tapes are expensive to produce; most are constructed by adding a conductive component to the adhesive, such as nickel fibers, carbon powder, or conductive salts. The adhesive properties of the tape are reduced as a result of adding the conductive material to the adhesive. An additional drawback to adding metal to the adhesive is the corrosion which often occurs when two different metals of the board and adhesive come into contact with each other. These metals can form essentially a battery whereby electrical current flows, corroding the metals.
The conductive adhesives are also not always sufficiently conductive to provide the desired protection. The static charge cannot be dissipated to ground quickly enough through an adhesive having a resistivity of 10.sup.7 -10.sup.9 ohm/square to protect the masked components. To be effective the resistivity of the conductive adhesive should be 10.sup.5 ohm/square or less for rapid charge dissipation. Perhaps the most serious drawback to conductive adhesives has been the propensity for these compounds to leave conductive residue on the surface of the board after removal. Conductive contamination from these tapes is a well-documented serious and costly problem that can render electronic components and assemblies inoperable.
Accordingly, it is important not only to protect selected components from conformal coating, but also to protect every electrical component electrically connected to the board from static electrical discharge.